Certain new types of vehicles, known as hybrid vehicles, employ a combustion engine coupled with a combination electric motor-generator in order to provide vehicle locomotion. In some of these hybrid or mild-hybrid powertrain systems, an electric motor-generator system replaces the conventional starter motor and alternator. When the hybrid vehicle is decelerating or is stopped, the fuel flow to the engine is shut off to improve fuel economy. While the hybrid vehicle is at a standstill, the engine is not turning. The motor-generator system of the hybrid vehicle is implemented to enable this fuel cutoff feature while minimally affecting drivability.
In a mild-hybrid powertrain with an automatic transmission, when the brake pedal is released after a stop, the motor-generator system spins up the engine, and creeps the vehicle forward, similar to a conventional vehicle with an automatic transmission. The engine combustion can be commenced after some period of time, or when the accelerator pedal is depressed. When the driver accelerates, the combustion engine restarts automatically and the hybrid vehicle can be driven in a conventional manner. When the combustion engine is running, the motor-generator acts as a generator to supply the vehicle's electrical power requirements, as well as recharging the on-board battery system. Whenever the combustion engine is off, the hybrid vehicle's electrical loads (fans, radio, etc.) are supported by the battery system and a DCDC converter. Though the term “Hybrid Powertrains” is often used to refer to hybrid-electric powertrains, other forms of hybridization (e.g., hydraulic) can be considered for this application.
While this new hybrid automotive design is advantageous from a fuel economy perspective, it can also present a need for additional design considerations in certain operational situations. One such situation is when the hybrid vehicle's engine shuts off as the vehicle comes to a stop on an incline. Since the combustion engine is typically shut off whenever the hybrid vehicle is stopped, the normal forward “creep” associated with the torque from the transmission in a standard vehicle is absent. Any vehicle facing uphill will have a greater tendency to roll backwards whenever the engine shuts off when compared to the same situation with the engine running. This is a rare event on a non-hybrid vehicle and only occurs if the combustion engine inadvertently stalls. On hybrid vehicles, an increase in brake pedal effort combined with a fast engine restart strategy on a brake release is commonly used to compensate for the loss of “creep” torque and manage backward movement of the vehicle. However, even with the best of the currently implemented techniques, there may be a slight backward rolling movement for hybrid vehicles idling or stopped on an incline.
In view of the foregoing, it should be appreciated that it would be desirable to provide methods and apparatus for minimizing the brake pedal effort required for controlling the backwards movement of a hybrid vehicle idling or stopped on an incline. Furthermore, additional desirable features will become apparent to one skilled in the art from the foregoing background of the invention and following detailed description of a preferred exemplary embodiment and appended claims.